1. Field of the Invention
This invention relates to video signal processing systems in which a video signal such as the composite color television signal of, for example, NTSC standards, is separated into luminance and chrominance informations.
2. Description of the Related Art
The complete color video signals now in use, a typical example of which is the composite color television signal S of NTSC standards, have the composite form of luminance information Y and chrominance information C, the latter further in the 90.degree.-apart 2-phase modulated form of the two color difference signals U and V (or I and Q) on a color subcarrier of frequency fsc. It can be written as S=Y+C=Y+U sin (2 fsc t)+Vcos(2 fsc t). From the frame frequency f.sub.F (30 Hz), field frequency f.sub.V (60 Hz) and horizontal scanning frequency f.sub.H, we have EQU fsc=(455/2) f.sub.H =(455/2)(525/2) f.sub.V =(455/2)525f.sub.F
For this reason, when the composite color television signal of the NTSC standards is sampled at a frequency equal to 4 times the frequency fsc of the color subcarrier, the sampled portions of the signal convey a series of four different classes of the informations in a distribution shown in FIG. 1. In more detail, the chrominance signal reverses in phase from line to line, and is extracted four samples in each period of the color subcarrier.
In order to retrieve from such a television signal, its luminance and chrominance components Y and C, the prior art makes use of such a forming filter as shown in FIG. 2 as the separator, wherein the arriving portion (Y-C) for the present line of the television signal is added to that portion for the preceding line which was delayed 1H in passing through a circuit 1, or the one (Y+C) by a first adder 2, because, as has been shown in FIG. 1, the chrominance signal in the same vertical position changes its phase 180.degree. between the present and preceding lines. So, the output of the adder 2 represents 2Y. Its level is then attenuated 1/2 by a first attenuator 3. Thus, the luminance signal Y is separated out. To separate out the chrominance signal C, the arriving television signal (Y-C) is routed to an inverter 4 and its output is added to the output of the 1H delay circuit 1 by a second adder 5. The output of the adder 5 which represents 2C is then attenuated 1/2 by a second attenuator 6.
It will be appreciated from the foregoing that the principle of such a conventional separator for the luminance and chrominance signals is based on the assumption that the picture element information in the form of a series of sampled portions of the composite color television signal has so strong a correlation in the vertical direction as to permit use of the picture element information of the preceding line for separating out the luminance and chrominance signals of the present line.
With such a conventional type, however, if a scene varies in brightness and color rapidly between the adjacent two of its sampled portions, these portions of the original scene will be reproduced with turbid color or dot hindrance or other deteriorations of the picture quality. Of these, the most serious is that the resolution drops extremely in the vertical direction particularly at the edges of things in the scene.
Attempts have been made to eliminate the above-described problem by using a digital delay circuit that enables the correlation of picture elements in both vertical and horizontal directions to be taken into account. But, any of a wide variety of the previous proposals has an alternate problem that the production cost becomes far higher than when the delay circuit is of the analog type.